Disk drive estimating fly height using a PLL tuned by a fly height capacitance

Abstract

A disk drive is disclosed comprising a disk, a head actuated over the disk, and control circuitry operable to estimate a fly height of the head by tuning a phase locked loop (PLL) in response to a fly height capacitance. In one embodiment, the PLL comprises a resistive component having a second terminal coupled to the fly height capacitance, a variable oscillator operable to generate a first oscillating signal applied to a first terminal of the resistive component to generate a second oscillating signal at the second end of the resistive component, and a phase detector operable to generate a control signal by comparing a phase of the first oscillating signal to a phase of the second oscillating signal. The control signal is applied to the variable oscillator to adjust a frequency of the first oscillating signal.

1. A disk drive comprising:
 a disk; 
 a head actuated over the disk; and 
 control circuitry operable to estimate a fly height of the head by tuning a phase locked loop (PLL) in response to a fly height capacitance. 
 
   
   
     2. The disk drive as recited in  claim 1 , wherein the PLL comprises:
 a resistive component comprising a first terminal and a second terminal, wherein the second terminal is coupled to the fly height capacitance; 
 a variable oscillator operable to generate a first oscillating signal applied to the first terminal of the resistive component to generate a second oscillating signal at the second end of the resistive component; and 
 a phase detector operable to generate a control signal by comparing a phase of the first oscillating signal to a phase of the second oscillating signal, wherein the control signal is applied to the variable oscillator to adjust a frequency of the first oscillating signal. 
 
   
   
     3. The disk drive as recited in  claim 2 , wherein the control signal adjusts the frequency of the first oscillating signal to maintain a target phase difference between the phase of the first oscillating signal and the phase of the second oscillating signal. 
   
   
     4. The disk drive as recited in  claim 3 , wherein the target phase difference is selected from the group consisting of zero degrees and 180 degrees. 
   
   
     5. The disk drive as recited in  claim 2 , wherein the control circuitry is further operable to convert the frequency of the oscillator signal into the estimate of the fly height. 
   
   
     6. The disk drive as recited in  claim 2 , wherein the control circuitry is further operable to convert the control signal into the estimate of the fly height. 
   
   
     7. The disk drive as recited in  claim 2 , further comprising a first transmission line coupling the second terminal of the resistive component to a surface of the head. 
   
   
     8. The disk drive as recited in  claim 7 , further comprising a second transmission line coupling the surface of the head to a ground. 
   
   
     9. The disk drive as recited in  claim 7 , further comprising a filter inserted between the second terminal and the surface of the head along a transmission path of the transmission line. 
   
   
     10. The disk drive as recited in  claim 9 , wherein the filter is implemented by varying a geometry of the first transmission line. 
   
   
     11. The disk drive as recited in  claim 2 , wherein:
 the head comprises a first and second capacitor plates; and 
 the fly height capacitance comprises a first capacitance formed between the first capacitor plate and the disk and a second capacitance formed between the second capacitor place and the disk. 
 
   
   
     12. The disk drive as recited in  claim 11 , further comprising:
 a first transmission line coupling the second terminal of the resistive component to the first capacitor plate; and 
 a second transmission line coupling a third terminal of the resistive component to the second capacitor plate. 
 
   
   
     13. The disk drive as recited in  claim 1 , wherein the fly height capacitance comprises a capacitance formed between a surface of the head and a surface of the disk. 
   
   
     14. The disk drive as recited in  claim 1 , further comprising:
 an actuator operable to adjust the fly height of the head; 
 a comparator operable to compare the estimate of the fly height to a target fly height to generate a fly height error; and 
 a fly height controller operable to generate a control signal applied to the actuator in response to the fly height error. 
 
   
   
     15. A method of estimating a fly height of a head over a disk in a disk drive, the method comprising tuning a phase locked loop (PLL) in response to a fly height capacitance. 
   
   
     16. The method as recited in  claim 15 , wherein the PLL comprises a resistive component comprising a first terminal and a second terminal, wherein the second terminal is coupled to the fly height capacitance, the method further comprising:
 generating a first oscillating signal applied to the first terminal of the resistive component to generate a second oscillating signal at the second end of the resistive component; 
 generating a control signal by comparing a phase of the first oscillating signal to a phase of the second oscillating signal; and 
 adjusting a frequency of the first oscillating signal in response to the control signal. 
 
   
   
     17. The method as recited in  claim 16 , wherein the frequency of the first oscillating signal is adjusted to maintain a target phase difference between the phase of the first oscillating signal and the phase of the second oscillating signal. 
   
   
     18. The method as recited in  claim 17 , wherein the target phase difference is selected from the group consisting of zero degrees and 180 degrees. 
   
   
     19. The method as recited in  claim 16 , further comprising converting the frequency of the oscillator signal into the estimate of the fly height. 
   
   
     20. The method as recited in  claim 16 , further comprising converting the control signal into the estimate of the fly height. 
   
   
     21. The method as recited in  claim 16 , wherein a first transmission line couples the second terminal of the resistive component to a surface of the head. 
   
   
     22. The method as recited in  claim 21 , wherein a second transmission line couples the surface of the head to a ground. 
   
   
     23. The method as recited in  claim 21 , further comprising filtering the second oscillating signal to reduce a sensitivity of the fly height estimate to impedance variations in the first transmission line. 
   
   
     24. The method as recited in  claim 23 , wherein the filter is implemented by varying a geometry of the first transmission line. 
   
   
     25. The method as recited in  claim 16 , wherein:
 the head comprises a first and second capacitor plates; and 
 the fly height capacitance comprises a first capacitance formed between the first capacitor plate and the disk and a second capacitance formed between the second capacitor place and the disk. 
 
   
   
     26. The method as recited in  claim 25 , wherein:
 a first transmission line couples the second terminal of the resistive component to the first capacitor plate; and 
 a second transmission line couples a third terminal of the resistive component to the second capacitor plate. 
 
   
   
     27. The method as recited in  claim 15 , wherein the fly height capacitance comprises a capacitance formed between a surface of the head and a surface of the disk. 
   
   
     28. The method as recited in  claim 15 , further comprising:
 comparing the estimate of the fly height to a target fly height to generate a fly height error; and 
 adjusting the fly height of the head in response to the fly height error.